How jute stick fired boilers work
Jute sticks are the woody core left after the fibre is stripped from retted jute stalks, mainly in West Bengal, Assam and Bihar. They are light and hollow, with a bulk density of about 100 to 150 kg/m³, 10 to 15% moisture when sun-dried, 1 to 3% ash and a calorific value of about 3,800 to 4,200 kcal/kg. Volatile matter is high, so the sticks flame quickly and burn mostly above the bed. They are chopped and charged by hand or by conveyor onto fixed or inclined grates of smoke tube and small water tube boilers in jute mills, rice mills and food units. The flue gas passes the boiler, sometimes an economiser, and a cyclone or multicyclone before the ID fan and chimney.
Where the ID fan sits
- Fixed or inclined grate
- Boiler passes
- Economiser (if fitted)
- Cyclone or multicyclone
- ID fan
- Chimney
The fan sits after the cyclone, usually on boilers of 1 to 6 TPH. Jute stick ash is low, but flakes of char leave the furnace easily because they are so light, and a cyclone separates them poorly. Suction at the fan inlet is typically 80 to 150 mmWC. On boilers charged by hand through a firing door, the fan must keep the furnace at suction while the door is open, or flame and smoke come out at the fireman.
Gas at the ID fan
| Gas temperature, normal | 180 to 230°C, often with no economiser |
| Maximum or upset | Up to 280°C after a heavy charge or with soot-coated tubes |
| Dust at the fan | Low ash but light char flakes, sometimes glowing; a few hundred mg/Nm³ after a cyclone |
| Moisture and dew point | Water dew point about 45 to 55°C with sun-dried sticks; higher with sticks wet from the monsoon |
| Corrosive species | Minor; soot and tarry condensate if the gas is cooled near its dew point |
| Density at the design temperature | About 0.73 kg/m³ at 210°C, 61% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
ID Fan selection for jute stick fired boilers
| Blade form | Backward curved, or radial tipped where char loading behind a simple cyclone is heavy |
| Material of construction | Mild steel throughout; wear protection seldom needed |
| Drive and arrangement | Direct coupled arrangement 8, or belt driven on small units |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive on furnace draft, tuned for fast response |
| Also specified | Spark arrestor or settling chamber upstream, an inspection door and a casing drain |
What this ID fan has to cope with
Draft swings with each charge
Light sticks release their volatiles in a burst when they land on a hot bed. The gas volume jumps for several seconds and the furnace goes positive. A fan with a steep, stable curve, and a draft loop fast enough to follow the swing, keeps the firing door safe and stops the black puff at the chimney after every charge.
Char and sparks at the fan
Flaming char flakes are carried out of the furnace and may still glow at the cyclone outlet. They settle in the fan casing and ducts, where they can ignite soot and tarry deposits. A settling chamber or spark arrestor ahead of the fan, and a door and drain on the casing for regular cleaning, reduce the fire risk.
Soot on the boiler tubes
With high volatile matter and a short flame, poor mixing leaves soot on the tubes. Gas temperature at the fan creeps up between cleanings, the gas becomes lighter and its volume grows. The fan selection includes this rise, and the bearing and shaft design covers the hotter gas until the tubes are brushed.
Energy and control
Jute mills and small process units often run their boilers on one or two shifts with frequent load changes. A variable speed ID fan saves power at part load and follows charging far better than a hand-set damper. Adding an economiser where none exists recovers heat from gas that often leaves above 200°C, and lowers the gas volume at the fan, cutting fan power as well as fuel.
ID fan and FD fan in jute stick fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone, before the chimney | At the boiler front, feeding grate and overfire nozzles |
| What it handles | Flue gas with light char flakes | Ambient air with jute fibre |
| Temperature at the fan | 180 to 230°C, up to 280°C | Ambient, 20 to 40°C |
| Main design concern | Draft swings and sparks | Overfire air for volatiles |
| Control | Furnace draft | Steam pressure |
See also the Forced Draft (FD) Fan for jute stick fired boilers.
Questions buyers ask
Why does flame come out of the firing door on my jute stick boiler?
Jute sticks release their volatiles very quickly when charged, so the gas volume jumps and the furnace goes positive. The ID fan may be undersized or held at a fixed damper position. A correctly sized induced draft fan with a variable speed drive on furnace draft holds suction through each charge.
Does a jute stick fired boiler need a dust collector?
Yes. Ash is low, but light char flakes escape the furnace and would leave the chimney as sparks and black smuts that settle on nearby roofs. A cyclone or multicyclone, with a spark arrestor where needed, protects the ID fan and the surroundings and is usually needed to meet emission limits.
Is a radial blade ID fan needed for jute stick?
Not usually. Jute stick ash is low and not very abrasive, so a backward curved fan with better efficiency is suitable. A radial tipped wheel is used only where the fan follows a simple cyclone and char loading is heavy enough to build up on backward curved blades.
Information needed to quote an ID fan for jute stick fired boilers
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Charging method. Hand charging, conveyor or screw feeding, and the charging interval.
- Spark protection. Present collector and any spark arrestor or settling chamber before the fan.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.